FileStorageKey (Default) Creates Empty File Instead of Populating It with Data

[zachgibson]

Description

When using FileStorageKey<String>.Default, I’m seeing an issue where the file is created on disk with no contents, even though a default value is provided and used in memory. If I update that state the file then gets populated correctly.

Checklist

• I have determined whether this bug is also reproducible in a vanilla SwiftUI project.
• If possible, I've reproduced the issue using the main branch of this package.
• This issue hasn't been addressed in an existing GitHub issue or discussion.

Expected behavior

FileStorageKey<T>.Default with a non-nil default value creates a file on disk with the contents of default value.

Actual behavior

FileStorageKey<T>.Default with a non-nil default value creates an empty file on disk.

Reproducing project

FileStorageRepro.zip

Sharing version information

2.5.2

Destination operating system

macOS 15.3.1

Xcode version information

16.2

Swift Compiler version information

swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
Target: arm64-apple-macosx15.0

[mbrandonw]

Hi @zachgibson, this is currently by design. We do not eagerly write the initial value to the external system (in any strategy, including appStorage). The reason a file with empty bytes is being written is because we need a file on disk to observe changes to it via dispatch source.

Can you describe what problem this is causing for you so that we can evaluate?

But since this isn't an issue with the library I am going to convert it to a discussion. Let's continue the conversation over there!

[zachgibson]

I see @mbrandonw, this makes sense. I guess I'm just using it in an unexpected way.

My use-case is a Metal shader "REPL-like" app. When saving your shader externally Sharing picks that up and we re-compile/draw the new shader. So, when someone creates a new shader in the app I default the string to a sample shader program that has all the necessary boilerplate. This is where I am needing the file to have the contents initially.

[zachgibson]

I'm down to handle this in my own way but I haven't thought of a reliable mechanism to know when to update the state to cause it to be written to the file.

[zachgibson]

For now I'm creating the file and seeding the contents myself at creation time. @mbrandonw any considerations here or is this totally fine?